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 Post subject: exFAT quirks
PostPosted: Sun Nov 06, 2016 3:37 pm 
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exFAT quirks:
  • Windows will only mount the flash drive if the volume length in the boot sector matches the physical volume size unless the drive is partitioned.
  • Windows will mount the image/drive as read only if it only has a minimal upcase table (the first 128 ASCII UNICODE characters).
  • The eight extended boot sectors (for additional boot sector code) each require a 0xAA55 signature which means that the 4K of extra code cannot
    be contiguous and will require some extra work in placing and jumping around the signatures. Without the signatures the image/drive is perfectly usable,
    however "chkdsk" reports that the boot information is corrupt and cannot continue.
  • There are no "reserved sectors" (like ext2) in the volume. The alignment before the file allocation table and data region are mandatory (which can be
    zero in size) and the contents if any are undefined, which means that an implementation may modify as them necessary - rendering them useless for such
    purposes.
  • The wide character Unicode (16 bit) upcase table is compressed, however the specification shows as an example the compression (a simple RLE) of
    the minimal ascii Unicode upcase table which is not compressed in the upcase table as formatted by Windows.
  • A script is required to process the boot sector (and the additional boot sector code) to provide a checksum as well as a duplicate set of the first 12
    sectors. FASM has the capabilities to do this.

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Last edited by mikegonta on Mon Nov 07, 2016 1:12 pm, edited 3 times in total.

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 Post subject: Re: exFAT quirks
PostPosted: Mon Nov 07, 2016 4:05 am 
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Should this be on the exFAT wiki page?

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 Post subject: Re: exFAT quirks
PostPosted: Mon Nov 07, 2016 1:22 pm 
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mikegonta wrote:
exFAT quirks:
[*]The wide character Unicode (16 bit) upcase table is compressed, however the specification shows as an example the compression (a simple RLE) of
the minimal ascii Unicode upcase table which is not compressed in the upcase table as formatted by Windows.[/list]

Further investigation reveals that the additional compression will not save any medium space. Those implementations which only support ascii Unicode
can load the minimal upcase table in one sector and not have to decompress it (even thought decompression is an implementation requirement).
Of course, generating the table or doing the conversion in code is also possible.

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